As disclosed in e.g. publications WO 97/40899 and WO 97/40900 a solvent extraction settler arrangement is known which is adapted for hydrometallurgical liquid-liquid extraction processes and which typically comprises a mixing unit for preparing a dispersion from mutually immiscible solutions. A settler is arranged to separate solution phases from a dispersion fed from the feed end while the dispersion flows towards the discharge end of the settler. Elongated discharge launders are arranged at the discharge end of the settler to receive from the settler and discharge each solution phase that is separated from the dispersion in the settler. Each discharge launder includes a first end to which an outlet is arranged and a closed second end.
In the known technology the discharge launder has a same cross-sectional shape along the entire length of the discharge launder. In operation, as the launder collects solution flows of the solution from the settler along its entire length, and the whole volume is discharged from the launder from the outlet at the first end of the launder, the flow volume increases gradually along the length of the launder as the flow propagates towards the outlet. The discharge launder with a uniform cross-section is normally designed for the greatest flow volume to achieve a certain flow rate. The flow volume is greatest adjacent to the outlet and therefore the cross-section of the discharge launder is optimal only at a very short range of the launder.
In a launder with a constant cross-section the flow rate is lower at positions which are distant from the outlet in relation to other positions which are nearer the outlet whereby standing flow zones and eddies may occur. If the solution contains solids, crud accumulation may occur in these areas of standing zones and eddies. One disadvantage of the constant cross-section of the launder is also that the structure of the launder includes an excess of material which actually would not be needed.